1. Field of the Invention
The invention relates in general to power line carrier communication systems, and more specifically to distribution network power line carrier systems for performing distribution automation functions.
2. Description of the Prior Art
Power line carrier communication has been used for many years over high voltage transmission lines between the power generating site and power transmission switching site or distribution substation sites, for supervisory control purposes. Less common is communication over the power line conductors of a distribution network, from the distribution substation through the distribution transformers to the electrical load. Communicating over the distribution network power lines is more difficult than over the high voltage transmission lines, as the distribution lines, along with the large number of distribution transformers, present relatively poor signal transmission characteristics, and they introduce electrical noise and signal interference.
The recent increase in the desirability of performing certain distribution automation functions, such as automatic reading of utility meters, selective load control, performing load continuity checks, and the like, has made it attractive to use the distribution power line conductors for communications, but the problems connected therewith will have to be economically dealt with in order to provide a practical system.
U.S. Pat. Nos. 3,656,112; 3,702,460 and 3,815,119 and co-pending application Ser. No. 425,759, filed Dec. 18, 1973, all disclose some form of communication via the distribution network of an electrical utility. U.S. Pat. No. 3,656,112 discloses a communications system which uses a combination of an electric power line and a wireless link. The wireless link is used to bypass distribution transformers and other obstructions located in the transmission path. U.S. Pat. No. 3,702,460 discloses making the neutral conductor of the distribution power line available as a communication link by inserting a parallel resonant circuit between the neutral and ground, at each ground point in the system. The neutralground communication circuit bypasses distribution transformers. U.S. Pat. No. 3,815,119 uses the power line conductors on the secondary side of a distribution transformer to transfer meter readings associated with the secondary circuits to a common receiver point, where the readings are stored until they can be read, such as by a mobile unit which periodically travels near the receiver site and interrogates the site by a wireless link. The co-pending application Ser. No. 425,759, which is assigned to the same assignee as the present application, provides communication zones by using repeaters as frequency translators, isolating various sections of the distribution power line system by using different frequency bands.